KIEV, Ukraine (CNN) -- Dozens of armed men seized the regional government administration building and parliament in Ukraine's southern Crimean region Thursday and raised the Russian flag in a challenge to the Eastern European country's new leaders.

Crimea, a Black Sea peninsula with an ethnic Russian majority, is the last big bastion of opposition to the new political leadership in the capital, Kiev, after Ukrainian President Viktor Yanukovych's ouster Saturday.

The incident, coming a day after Russia ordered surprise military exercises on Ukraine's doorstep, has raised fears about the push and pull of opposing allegiances in a country sandwiched between Russia and the European Union.

There's a broad divide between those who support developments in Kiev -- where parliament was voting on an interim West-leaning, national unity government Thursday -- and those who back Russia's continued influence in Crimea and across Ukraine.

Yanukovych issued a defiant statement to Russian news agencies condemning the country's interim government and saying that everything happening now in the Ukrainian parliament is illegitimate, Russian state news agency RIA Novosti reported Thursday.

According to RIA Novosti, anonymous government sources said Thursday that he was in Russia and that Russian authorities have accepted his request for security. A warrant has been issued for his arrest in Ukraine.

Yanukovych will give a news conference Friday in the southern Russian city of Rostov-on-Don, Russian state media reported. If so, it would be the first time he's been seen in public since fleeing Kiev. CNN has not independently confirmed Yanukovych's whereabouts.

New Prime Minister appointed

Back in Ukraine's capital, lawmakers approved opposition leader Arseniy Yatsenyuk of the Batkivschina, or Fatherland, party as Prime Minister.

Yatsenyuk, who has served as economic minister and foreign minister in past governments, told reporters that Yanukovych "is no longer the President, he is the person under investigation and accused of crimes against humanity," state news agency Ukrinform reported.

The new Prime Minister told parliament that he cannot promise to turn things around quickly and that there is likely to be pain in the short term, as the cash-strapped country seeks to get back on track.

International Monetary Fund chief Christine Lagarde said Thursday that her organization was ready to respond to a request for assistance from Ukrainian authorities and would send a fact-finding team to Ukraine to begin preliminary discussion of reforms the country needs. "We are also discussing with all our international partners -- bilateral and multilateral -- how best to help Ukraine at this critical moment in its history," she said.

At a meeting of NATO's defense ministers in Brussels, Belgium, NATO Secretary General Anders Fogh Rasmussen urged the new Ukrainian leadership "to continue its efforts to establish an inclusive political process that reflects the democratic aspirations of the entire Ukrainian people."

Earlier in the day, he also urged Russia to act with caution. "I'm concerned about developments in #Crimea. I urge #Russia not to take any action that can escalate tension or create misunderstanding," he tweeted.

In turn, the Russian Foreign Ministry said the Kremlin will "give a tough and uncompromised response to violations of compatriots' rights by foreign states."

U.S. concerns about Russian military exercises

The Russian military exercises near the Ukraine border are making U.S. military and intelligence agencies concerned that Russia may be positioning ground forces to be able to move across the border into Ukraine if Moscow issued such orders, U.S. officials told CNN.

A senior U.S. official familiar with the most recent administration assessment told CNN that now that the Russians "have brought troops out of garrison," they could potentially move "quite quickly" once an order comes.

At that point, the official says, the U.S. assessment is that its "warning time" that Russian forces were on the move might be so short, it would be difficult for the United States to move diplomatically to try to stop it.

However, the U.S. military and intelligence agencies still conclude that for now, Russia does not intend to move its military forces into Ukraine, despite rising rhetoric.

At present, the United States believes the exercises are part of the "message being sent" by President Vladimir Putin, to both Ukraine and the United States, that he can move his military into Ukraine to protect Russian interests if he chooses to do so, the official said.

Russian Defense Minister Sergey Shoigu said Wednesday that the exercises were to check "combat readiness."

Two U.S. officials earlier said that intelligence suggests Russia is "repositioning" up to half a dozen Russian ships near the Ukrainian port city of Sevastopol in case they're needed to respond if Russian interests in Crimea are threatened.

No negotiations

It was not immediately known who was occupying the buildings in Simferopol, the Crimean regional capital. The head of the region, Premier Anatolii Mohyliov, told CNN the gunmen were refusing to speak with him, telling him he had no authority.

The men, who stormed the building early Thursday morning, had made no demands, and it was not clear what they wanted, he said.

He said there are no civilians in the building and "the situation is under control." He added that government security forces, which were outside the buildings, would not use force or weapons to take over the buildings.

"All police in Ukraine have been ordered to be prepared," acting Ukrainian Interior Minister Arsen Avakov wrote on his Facebook page. "Orders have been issued to create a cordon around the Parliament in Crimea and to avoid shooting and violence."

A witness, who gave his name only as Maxim, said he saw the armed men run into the building and kick out the police. "Inside, nobody knows what is going on inside at the moment. We only saw the building being taken over," he said.

He said the men took bags containing anti-tank weapons, sniper rifles, assault rifles and handguns from buses into the building.

Rasmussen, NATO's secretary general, described the seizure of the regional government administration building and parliament as "dangerous and irresponsible."

UK Prime Minister David Cameron, speaking alongside German Chancellor Angela Merkel in London, said that both nations "support a united and democratic Ukraine."

He added, "We are particularly concerned by the situation in Crimea. Every country should respect the territorial integrity and sovereignty of the Ukraine. Russia has made that commitment, and it's important that Russia keeps its word. The world will be watching."

Military vehicles seen

Meanwhile, Violetta Lisina, press secretary to the Crimean Prime Minister, wrote on her Facebook page that there were seven armored personnel carriers for a period outside the village of Ukromnoye earlier Thursday.

She said the vehicles were Russian, from the Russian Black Sea Fleet, which is based at Sevastopol. The village is roughly 9 miles northwest of Simferopol, and about an hour north of Sevastopol.

In response to the reports, Russia's Foreign Ministry said in a prepared statement that any movement by Black Sea Fleet armored vehicles was in "strict compliance" with its agreements with Ukraine.

Ukraine's Foreign Ministry summoned Russia's charge d'affaires Thursday morning and gave him a note requesting that Russian Black Sea military units based in Sevastopol remain on base, Ukrinform reported.

Secession fears

Concerns are building that the tensions in the Crimean region might escalate into a bid for separatism by its Russian majority.

The speaker of the Crimean parliament, Vladimir Konstantinov, said Thursday that the people of Crimea wanted a referendum on increased autonomy and authority for the region. He spoke in the Crimean parliament building in an interview by a local TV station, retransmitted by Ukraine's Channel 5.

On Wednesday, Konstantinov dismissed local media reports that secession might be under discussion as "rumors," and he urged residents to not be provoked.

Crimea, an autonomous republic, was handed to Ukraine by the Soviet Union in 1954. Just over half its population is ethnic Russian, while about a quarter are Ukrainians and a little over 10% are Crimean Tatars, a group oppressed under former Soviet leader Joseph Stalin.

Many are struggling to come to grips with the rapid political upheaval.

In Sevastopol, residents told CNN they were angry that Yanukovych had been forced out and fear that they will be oppressed by the country's new leaders.

Scuffles broke out Wednesday when rival groups, one pro-Russian and the other supporting the new authorities in Kiev, rallied in front of the Crimean parliament building in Simferopol.

Russia's Foreign Ministry has accused Ukraine's lawmakers of discriminating against ethnic Russians by excluding them from the reform process.

New government

In the capital, Kiev, Ukraine's parliament voted in a new government Thursday after protest leaders named the ministers they want to form a new Cabinet.

All members of the new government have now been appointed, with the exception of the defense and foreign ministers.

Acting ministers are filling these two roles until presidential elections on May 25, because under Ukraine's constitution, they must be named by the president. Once that happens, they will be confirmed by parliament.

In a display of people power, the announcement of Yatsenyuk and candidates for other key ministries was made Wednesday after protest leaders addressed crowds on Independence Square, the heart of the protests.

Local elections are also due on May 25. Opposition leader and former heavyweight boxing champion Vitali Klitschko will run for the presidency, his press secretary said.

Last week, bloody street clashes between demonstrators and security forces killed more than 80 people, the deadliest violence in the country since it gained independence when the Soviet Union collapsed 22 years ago.

Yanukovych's decision to scrap a European Union trade deal in favor of one with Russia prompted the protests, which began in November.

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